docs: def autoconfig_url_from_preferences():
      """
      Get the PAC ``AutoConfigURL`` value from the macOS System Preferences.
      This setting is visible as the "URL" field in
      System Preferences > Network > Advanced... > Proxies > Automatic Proxy Configuration.
  
      :return: The value from the registry, or None if the value isn't configured or available.
          Note that it may be local filesystem path instead of a URL.
      :rtype: str|None
      :raises NotDarwinError: If called on a non-macOS/OSX platform.
      """
      if not ON_DARWIN:
          raise NotDarwinError()
  
      try:
          config = SystemConfiguration.SCDynamicStoreCopyProxies(None)
      except AttributeError:
          return  # Key or value not found.
  
      if all(('ProxyAutoConfigEnable' in config,
              'ProxyAutoConfigURLString' in config,
              not config.get('ProxyAutoDiscoveryEnable', 0))):
          # Only return a value if it is enabled, not empty, and auto discovery is disabled.
          return str(config['ProxyAutoConfigURLString'])